However, as CD4+ T cells are the major mediators of CNS injury during the course of EAE, in studies of human MS, in which conspicuous CD8+ T cells are infiltrated throughout lesions at all stages, whereas CD4+ T cells are sparse or even absent, an added challenge for determining the precise role of CD4+ T cells is ascertaining whether these effectors are more critical in disease initiation than in established MS (11). The gene discussed is CD8A; the disease is myeloid sarcoma.